With the Canon Film SLR 28-80 mm lens with sunfilter (I think it has zero impact on focal length) what should be my aperture settings, if I have to take pictures of people in crowd and restaurant to ensure only my subject gets the focus. How to calculate it?

Yes I have been taking it by trial and error and seems like a dumb with all the calculations I see on Google. I am not able to tell what I should be setting for a person 15 feet away in the crowd to a person 5 feet away (stationary and moving slowly and in another case subject is my nephew on a moped). The filter is daylight SKYLIGHT filter 58 mm. My problem is being Film camera I am not able to remember the settings to understand the technique better any ideas on that as well.

3 Answers
3

There's not enough information here to answer, because we're missing several important things.

First, does your film SLR have metering? If so, is it "through the lens metering"? If it is a recent camera (1980s or after), the answer is almost certainly "yes", which makes the answer to your question "just do what the camera's meter tells you".

The next bit of missing information is detail on the filter itself. Do you mean a clear "daylight" filter (usually used for lens protection)? In that case, there is no impact. But you might have a neutral density filter, which is shaded and reduces the brightness. If so, you will need to know how strong (dark) the filter is — usually rated in "stops". If the camera does have TTL metering, this will be taken into account automatically.

Finally, we're missing all the other exposure parameters. What ISO film? What shutter speed? And, not to forget: how bright is the scene? The overall exposure is calculated from all of these things, so you can't set the aperture in isolation.

And all of that said, I notice that your concern is getting only your subject in focus, with a reduced depth of field. If that is your concern, and the camera does have automatic metering, set the aperture to as wide as you can (lower numbers) and then set the shutter speed to match for the ISO of film you are using. See How can I maximize the "blurry background, sharp subject" (bokeh) effect? for more on this technique in particular.

If you have a neutral density filter, that might be important here, because it will let you use those wide apertures with a longer shutter speed, so if the light is bright enough to exceed your camera's fastest shutter, or if you want to use a long shutter speed for artistic reasons, it will be useful. Otherwise, it doesn't matter.

Start by trying with as low of an aperture number as possible. If the depth of field is too shallow (can't get subject completely in focus), then try increasing it, but there is far more to getting the background blurry and the subject clear. You also want to use the longest focal length you can, be as close to the subject as you can and have the background proportionally farther away from the subject than the camera, however many of these are contradictory (you need a shorter focal length as you get closer to the subject).